Scanning photoelectron microscope

ABSTRACT

A scanning photoelectron microscope comprises a stage on which a sample is placed in a state in which gas around the sample is present, a light source emitting light of a wavelength capable of causing photoelectrons to be emitted from the sample, an optical system for condensing the light from the light source on the sample, scanning means for scanning the sample and the light relative to each other, and detecting means capable of applying positive potential to the sample, and detecting the photoelectrons created from the sample by the condensing, through the gas.

This is a continuation of application Ser. No. 08/220,173 filed Mar. 30,1994 U.S. Pat. No. 5,446,282.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a scanning photoelectron microscope, andparticularly to a scanning photoelectron microscope for observingtherethrough a sample having much gas emission, a sample containingwater, etc.

2. Related Background Art

In recent years, attention has been paid to a scanning photoelectronmicroscope as an apparatus for observing therethrough the chemicallycoupled state of the surface of a sample.

The scanning photoelectron microscope has scanned a sample placed in avacuum state higher than 10⁻¹ Pa with an energy beam (such as X-rays orultraviolet rays), has processed photoelectrons created from the sampleby this scanning in the following manner and has displayed the result ona CRT.

That is, the photoelectrons created from the sample have been detectedby a detector comprising a combination of a microchannel plate or ascintillator and a photoelectronic multiplier tube.

The photoelectrons detected by the detector have been stored as an imagesignal in an image memory in synchronism with the aforementionedscanning, and have been displayed on the CRT.

Also, when it is desired to observe photoelectrons having desiredenergy, an electron energy analyzer has been disposed between the sampleand the detector, and only the photoelectrons having desired energy havebeen taken out by this electron energy analyzer, whereafter thephotoelectrons having desired energy have been detected by the detector.

By the prior-art detector, however, the photoelectrons created from thesample can be detected only in the aforementioned high vacuum state, andthis has led to a problem that a sample having much gas emission and asample containing water cannot be observed.

SUMMARY OF THE INVENTION

So, the present invention has as its object the provision of a scanningphotoelectron microscope capable of detecting even photoelectrons from asample having much gas emission or a sample containing water.

The present invention includes a stage on which a sample is placed in astate in which gas around the sample is present, a light source emittinglight of a wavelength capable of causing photoelectrons to be emittedfrom said sample, an optical system for condensing the light from saidlight source on said sample, scanning means for scanning said sample andsaid light relative to each other, and detecting means capable ofimparting positive potential to said sample, and detectingphotoelectrons created from said sample by said condensing, through saidgas.

A preferred embodiment of the present invention further includes asample chamber for containing said sample and said detecting meanstherein and keeping them air-tight to the surroundings, a window forintroducing said light into said sample chamber, and control means forcontrolling said sample chamber to a gas atmosphere of predeterminedpressure.

Also, the present invention includes a stage on which a sample is placedin a state in which gas around the sample is present, a light sourceemitting light of a wavelength capable of causing photoelectrons to beemitted from said sample, an optical system for condensing the lightfrom said light source on said sample, scanning means for scanning saidsample and said light relative to each other, electron energy selectingmeans for selecting only those of the photoelectrons created from saidsample which have predetermined energy, detecting means capable ofimparting positive potential to said sample, and detecting thephotoelectrons selected by said electron energy selecting means, throughsaid gas, and a sample chamber capable of containing said sample, saidelectron energy selecting means and said detecting means therein so asto keep them air-tight to the surroundings, and introducing said lightinto said sample.

According to a preferred embodiment, when the ionization cross sectionof said gas by electrons is G and the molecule number density of saidgas is N, said electron energy selecting means is disposed at a locationwithin a value L found by the following equation from said sample towardthe optical axis of said condensing optical system:

    L=1/(N·σ)

More preferably, a preferred embodiment of the present inventionincludes control means for controlling said sample chamber to a gasatmosphere of predetermined pressure.

In the present invention, light (an electromagnetic wave of a shortwavelength less than a microwave) is applied to the sample in the gasatmosphere, whereby photoelectrons are emitted from the sample. As iswell known, photoelectrons are obtained by an electromagnetic wave of ashort wavelength less than a microwave, and especially, a wavelengthshorter than ultraviolet rays is advantageous. The light from whichphotoelectrons are obtained is distinguished from corpuscular rays suchas electron rays. This photoelectron, which it flies toward a detector(electrode) given positive potential while being accelerated, collideswith a gas molecule between the sample and the detector (electrode) andionizes the molecule. As a result, the electron of the photoelectron isamplified to two. By repeating this step, the electron of thephotoelectron is further amplified. This is called the electronmultiplying action.

This amplified photoelectron is detected by the detector (electrode) andbecomes a photoelectron signal.

The amplification degree of the photoelectron is obtained fromTownsend's first coefficient, and depends on the pressure of the gas,the kind of the gas, the potential of the detector and the distancebetween the sample and the detector. Therefore, the combination of theseis determined by the photoelectron signal. For example, the sample isplaced in oxygen gas of 1330 Pa (10 Torr), light of a wavelength 254 nmis applied to the sample, and a voltage of about 500 V is applied to thedetector spaced apart by about 10 mm from the sample, whereby thephotoelectron from the sample can be detected.

Also, when it is desired to observe a photoelectron having desiredenergy, when the ionization cross section of the gas by an electron is σand the molecule number density of the gas is N, the electron energyselecting means is disposed at a location within a value L found by thefollowing equation from the sample toward the optical axis of thecondensing optical system. The following equation is for finding thedistance by which the photoelectron advances its energy without losing(amplifying) the energy.

    L=1/(N·σ)

Thereby, without the loss of the energy of the photoelectron createdfrom the sample, this photoelectron can be directed to the electronenergy selecting means.

The electron energy selecting means selects only a photoelectron havingdesired energy. The detecting means detects this selected photoelectronthrough a gas having the electron multiplying action. Thereby, thephotoelectron having desired energy can be detected without the samplechamber being kept in high vacuum and therefore, a sample having muchgas emission, a sample containing water, etc. can be observed.

Also, the control means controls the interior of the sample chamber in agas atmosphere having the electron multiplying action of predeterminedpressure. Therefore, the pressure of the sample chamber can be set inaccordance with the sample to be observed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a scanning photoelectron microscopeaccording to a first embodiment of the present invention.

FIG. 2 is a schematic diagram of a combination of a scanningphotoelectron microscope according to a modification of the firstembodiment and a scanning optical microscope.

FIG. 3 is a diagram showing a second modification of the firstembodiment.

FIG. 4 is a schematic diagram of a scanning photoelectron microscopeaccording to a second embodiment of the present invention.

FIG. 5 shows the details of an electron energy analyzer 23.

FIG. 6 is a flow chart of a CPU 24.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic diagram of an embodiment of a scanningphotoelectron microscope according to the present invention.

In FIG. 1, a beam of light la from a light source 1 is caused to passthrough an aperture 3 by a condenser lens 2, is caused to enter ascanning mirror 4 by a drive unit 16 and is introduced into a samplechamber 6 into which gas has been introduced, by the reflection of thescanning mirror 4 through a window 5. The beam of light introduced intothe sample chamber 6 is condensed on a sample on a stage 8 by anobjective lens 7.

The beam of light is caused to scan the surface of the sampletwo-dimensionally (in X-direction and Y-direction) by the scanningmirror 4, as required. Two scanning mirrors 4 for X-direction andY-direction, respectively, for thus scanning the beam of light areusually prepared, but such construction is well known and therefore, inFIG. 1, for the sake of convenience, only one mirror 4 is shown. Forexample, of course, X-direction may be scanned by the scanning mirrorand in Y-direction, the surface of the sample may be moved, or thesurface of the sample may be moved two-dimensionally.

A photoelectron emitted from the sample by the application of the lightthereto ionizes the gas in the sample chamber 6, whereby the multipliedelectron is captured by a detector 9 (electrode) of positive potentialrelative to the sample. That is, the detector 9 is maintained atpositive potential by a power source 10 (the sample is earthed), andsuch a distance and voltage that no discharging will take place betweenthe detector and the sample are determined. The sample chamber 6 is keptat predetermined pressure by controlling a gas introduction unit 11 andan evacuation unit 12 by a gas control unit 13.

A photoelectron signal is displayed on a monitor 15 through an amplifier14 in synchronism with the mirror scanning. With such a construction,the sample interchange door, not shown, of the sample chamber 6 isopened, and then the sample is placed onto the stage 8 and the sampleinterchange door is closed, whereafter the gas control unit 13 isoperated to control the gas introduction unit 11 and the evacuation unit12, thereby keeping the interior of the sample chamber 6 atpredetermined pressure. Thereafter, the light from the light source 1 isemitted (this is accomplished by the changeover from the turn-off to theturn-on of the light source, or by the changeover from the closing tothe opening of a shutter, not shown).

The beam of light la on the sample is caused to scan two-dimensionallyby the drive unit 16, whereby the photoelectron emitted from the sampletravels toward the detector 9, but is captured by the detector 9 with anelectron created as a result of the photoelectron colliding with the gasto thereby ionize the gas, and is amplified by the amplifier 14,whereafter it is inputted to the monitor 15 and becomes the calescencepoint signal of the monitor 15, whereby the two-dimensionalphotoelectron image of the sample is displayed on the monitor 15.

FIG. 2 shows a modification comprising a combination of the scanningphotoelectron microscope shown in FIG. 1 and a scanning opticalmicroscope, and in FIG. 2, the same members as those in FIG. 1 are giventhe same reference numerals and need not be described. The scanningmirror 4 in FIG. 1 is constructed as a half-transmitting scanning mirror4' using a half mirror, and the reflected light from the sample ispassed through an aperture 30 and a condenser lens 17 and is detected bya photodetector 18. The detection signal is amplified by an amplifier19, whereafter it enters the monitor 15 and is displayed as a sampleimage on the monitor 15 in synchronism with the scanning of thehalf-transmitting scanning mirror 4'. The outputs of the amplifiers 14and 19 are superposed one upon the other, whereby an optical image and aphotoelectron image are obtained at a time. Of course, the output of theamplifier 14 and the output of the amplifier 19 may be selected by achangeover switch, not shown, which is provided in the monitor 15, andthe optical image and the photoelectron image may be alternativelydisplayed.

FIG. 3 shows a second modification in which the beam of light la is notscanned, but the stage 8 is driven two-dimensionally by a stage driveunit 20 and the sample is scanned relative to the beam of light la andthe sample is disposed in the atmosphere, the photoelectron signal iscaptured by the detector 9, is amplified by the amplifier 14 and isdisplayed on the monitor 15 in synchronism with the scanning of thesample.

In FIG. 3, the beam of light la from the light source is a beam of lightdifficult to scan by a mirror, for example, X-rays from synchrotronradiation or the like, and the mirror 7 is a toroidal X-ray mirror or aFreznel zone plate. A beryllium X-ray window is used as the window 5.

Also, in the above-described embodiments, the sample is placed in thesample chamber 6, but it is known that photoelectrons are also obtainedin the atmosphere, and it is not requisite to confine the sample in thepressure-controlled sample chamber 6.

FIG. 4 is a schematic diagram of a scanning photoelectron microscopeaccording to a second embodiment of the present invention.

In FIG. 4, the same members as those in FIG. 1 are given the samereference numerals and need not be described.

In FIG. 4, a synchrotron light source 1' is a light source capable ofemitting lights of wavelengths from the infrared range to theultraviolet range. A wavelength selection spectroscope 22 selects onlythe light of necessary wavelength from the beam of light 1a from thesynchrotron light source 1' and causes it to enter the condenser lens 2.In the second embodiment, the wavelength selection spectroscope 22causes only X-rays of wavelength 1 nm to enter the condenser lens 2.

A reflecting mirror 4a reflects the beam of light 1a and causes it topass through an X-ray transmitting window 5a of thin diamond film, andintroduces it into the sample chamber 6 into which gas has beenintroduced. In the present embodiment, the sample chamber 6 has a volumeof 1 m³. Also the arrangement of the optical system takes the absorptionof X-rays by the gas into consideration.

The stage 8 is movable two-dimensionally (in X-direction and Y-directionin FIG. 4), and in the second embodiment, by the two-dimensionalmovement of the stage 8, the beam of light 1a condensed on the sample 21is caused to scan two-dimensionally on the sample 21.

Photoelectrons created from the sample 21 by this scanning enter anelectron energy analyzer 23. The electron energy analyzer 23 passestherethrough only those of photoelectrons created from the sample 21which have necessary energy, and can be adjusted in Z-direction in FIG.4 so that it can be disposed at a location whereat the energy of thephotoelectrons is not lost (amplified) by the gas atmosphere in thesample chamber 6 (the details of this will be described later). Also, inthe present embodiment, an electron energy analyzer of the retardingfield type is used as the electron energy analyzer 23.

FIG. 5 shows the details of the electron energy analyzer 23 of FIG. 4,and the electron energy analyzer 23 is comprised of a grid 23a, a grid23b and a power source 23c. The power source 23c gives a voltage to thegrid 23a. The grid 23b drops photoelectrons not enough to satisfynecessary energy to the earth.

Turning back to FIG. 4, the photoelectrons passed through the electronenergy analyzer 23 collide with gas molecules in the sample chamber 6,and are detected by a detector 9a which is at positive potentialrelative to the sample 21. The detector 9a uses a ring-like metal (inthe present embodiment, copper) and is disposed so as not to interceptthe beam of light 1a, and is given a voltage so that the potentialdifference thereof from the grid 23a may not exceed 400 V.

A CPU 24 effects the adjustment of the electron energy analyzer 23 inZ-direction through a motor, not shown, and controls the gasintroduction unit 11, the evacuation unit 12 and a stage controller 25.

The evacuation unit 12 evacuates the sample chamber 6. The gasintroduction unit 11 introduces gas into the sample chamber 6, and inthis second embodiment it introduces helium (He).

The stage controller 25 moves the stage 8 two-dimensionally through amotor, not shown, as previously described.

In the second embodiment, X-rays of wavelength 1 nm is used andtherefore, the synchrotron light source 1' to the reflecting mirror 4aare covered with a cover 26 of lead.

The observing operation of the scanning photoelectron microscopeconstructed as described above will hereinafter be described.

FIG. 6 is a flow chart for the observation of the sample 21, anddescription will hereinafter be continued with reference to this flowchart.

The CPU 24 controls the pressure in the sample chamber 6 to the pressureindicated by the operator (step 101).

For example, when the sample chamber 6 is to be set to 500 Pa, the CPU24 evacuates the sample chamber 6 rendered into the atmospheric pressureby the interchange of the sample 21 to the order of 100 Pa by theevacuation unit 12. The CPU 24 introduces helium by the gas introductionunit 11 until the pressure in the sample chamber 6 becomes 500 Pa.Thereby, the CPU 24 can render the interior of the sample chamber 6 intoa helium atmosphere of 500 Pa.

The CPU 24 calculates the position of the electron energy analyzer 23 onthe basis of the set pressure (step 102).

The CPU 24 first finds the molecule number density (number/m³) of thehelium (He) gas.

The state equation of the gas is expressed by the following equation:

    P·V=nkt,

where P is the pressure (Pa), V is the volume (m³) of the sample chamber6, n is the molecule number of the gas in the sample chamber 6, isBoltzman's constant (JK⁻¹), and T is temperature (K).

Since the molecule number density N (number/m³) is n/V, the aboveequation can be modified as follows. (The molecule number density of thegas does not depend on the kind of the gas.)

    N=n/V=P/kT

If the temperature in the sample chamber 6 is 300 K (27° C.), aspreviously described, the volume of the sample chamber 6 is 1 m³ andBoltzman's constant is 1.380662×10⁻²³ (JK⁻¹) and therefore, thesenumerical values are substituted for the above equation to therebycalculate

    N=500/(300×1.380662×10.sup.-23)

and find

    N=1.207×10.sup.23 (number/m.sup.3).

The CPU 24 then calculates the position of the electron energy analyzer23 from the equation below. The equation below is for finding thedistance by which the photoelectron advances its energy without losing(amplifying) the energy.

    L=1/(N·σ),

where σ is the ionization and excitation cross section (m²) of the gasby the electron, and when the gas is helium gas in the requiredphotoelectron energy area, σ is 3.6×10⁻²¹ (m²) at maximum and therefore,

    L=1/(1.207×10.sup.23 ×3.6×10.sup.-21)

is calculated to thereby find

    L≃0.0023 (m)=2.3(mm).

The CPU 24 adjusts the position of the electron energy analyzer 23 inthe direction of the optical axis (step 103).

The CPU 24 adjusts the grid 23a of the electron energy analyzer 23 so asto assume a position spaced apart by 2.3 (mm) from the sample 21 alongthe optical axis (Z-direction). The grid 23b is adjusted so as to not tocontact with the sample 21.

The CPU 24 moves the stage 8 two-dimensionally and scans the sample 21with X-rays (step 104).

The wavelength selection spectroscope 22 passes therethough only X-raysof wavelength 1 nm of the beam of light 1a emitted from the synchrotronlight source 1' and therefore, the X-rays of wavelength 1 nm arecondensed on the sample 21.

When the X-rays are condensed on the sample 21, the CPU 24 moves thestage 8 two-dimensionally through the stage controller 25. By thetwo-dimensional movement of the stage 8, the sample 21 is scanned withthe beam of light 1a (step 105). A photoelectron corresponding to thisscanning position is created from the sample 21.

The CPU 24 makes the detector 9a detect the photoelectron created fromthe sample 21 (step 106).

The grid 23a of the electron energy analyzer 23, as previouslydescribed, is at a location spaced apart by 2.3 (mm) from the sample 21along the optical axis (Z-direction). Therefore, the photoelectronscreated from the sample 21 enter the electron energy analyzer 23 withoutlosing (amplifying) their energy. The electron energy analyzer 23selects and passes therethrough only those of the photoelectrons whichhave necessary energy. In the second embodiment, an electron energyanalyzer of the retarding field type is used as the electron energyanalyzer 23 and thus, when as described above, the grid 23a is at alocation spaced apart by 2.3 (mm) from the sample 21, a voltage of theorder of -400 V is applied with a discharge voltage taken into account.

The photoelectrons passed through the electron energy analyzer 23collide with the molecules of helium in the sample chamber 6 and thethereby amplified. These photoelectrons are detected by the detector 9a.

The CPU 24 makes the amplifier 14 amplify the photoelectrons detected bythe detector 9a as previously described, and causes them to be inputtedas an image signal to the monitor 15. Thus, the monitor 15 displays thephotoelectron image of the sample 21.

Also, when it is desired to change the pressure in the sample chamber 6and the kind of the gas, return can be made to the step 101. In thesecond embodiment, the photoelectrons are amplified by helium, any gassuch as steam or nitrogen can be applied.

In the following, the position of the electron energy analyzer 23 whennitrogen (N₂) is used as a modification of the second embodiment isfound. In this modification, the pressure in the sample chamber 6 is 100Pa. From the aforementioned state equation of the gas, the moleculedensity number N when the pressure in the sample chamber 6 is 100 Pa isN=2.41×10²² (number/m³). The maximum value of the ionization andexcitation cross section σ of nitrogen (N₂) by photoelectron is

    σ=2.6×10.sup.-20 (m.sup.2)

and therefore, this is substituted for L=1/(N·σ) to thereby calculate

    L=1/(2.41×10.sup.22 ×2.6×10.sup.-20),

thus finding

    L≃1.60 (mm).

From this, the grid 23a can be adjusted to a location distant by 1.60(mm) from the sample 21.

In the present embodiment, X-rays of wavelength 1 nm is used as thelight source, but X-rays of other wavelength or ultraviolet rays of awavelength of the order of 200 nm may also be used.

Also, in the present embodiment the stage 8 is moved two-dimensionallyto thereby scan the beam of light 1a condensed on the sample 21 and thesample 21, but alternatively, the reflecting mirror 4a may be caused toscan two-dimensionally. That is, as is well known, two mirrors forX-direction and Y-direction, respectively, can be used and design can bemade such that the respective mirrors are caused to scan.

Also, where the sample 21 is electrically conductive, photoelectrons notenough to satisfy the necessary energy can be dropped to the earththrough the sample 21 and therefore, the grid 23b can be eliminated.

What is claimed is:
 1. A scanning photoelectron microscopecomprising:(a) a stage on which a sample is placed in a state in whichgas is present around the sample; (b) a light source emitting light of awavelength capable of causing photoelectrons to be emitted from thesample; (c) a condensing optical system for condensing the light fromsaid light source on the sample; (d) a scanning system for scanning thesample and said light relative to each other; and (e) a detector capableof applying positive potential to the sample, and detectingphotoelectrons created from the sample by said condensing, through saidgas;said gas multiplying the photoelectrons from the sample.
 2. Ascanning photoelectron microscope according to claim 1, furthercomprising:(a) a sample chamber for containing the sample and saiddetector therein and keeping them air-tight to the surroundings; (b) awindow for introducing said light into said sample chamber; and (c) acontroller for controlling said sample chamber to a gas atmosphere ofpredetermined pressure.
 3. A scanning photoelectron microscope accordingto claim 1, wherein said light is X-rays, and said scanning is effectedby movement of said stage.
 4. A scanning photoelectron microscopeaccording to claim 1, further comprising:(a) a condensing lens systemfor condensing reflected light from the sample; and (b) a lightreceiving member for detecting the reflected light condensed by saidcondensing lens system.
 5. A scanning photoelectron microscopecomprising:(a) a stage on which a sample is placed in a state in whichgas is present around the sample; (b) a light source emitting light of awavelength capable of causing photoelectrons to be emitted from thesample; (c) a condensing optical system for condensing the light fromsaid light source on the sample; (d) a scanning system for scanning thesample and said light relative to each other; (e) an electron energyselecting device for selecting only those of the photoelectrons emittedfrom the sample which have predetermined energy; (f) a detector capableof imparting positive potential to the sample, and detecting thephotoelectrons selected by said electron energy selecting device,through said gas, said gas multiplying the photoelectrons from thesample; and (g) a sample chamber capable of containing the sample, saidelectron energy selecting device and said detector therein so as to keepthem air-tight to the surroundings, and introducing said light onto thesample.
 6. A scanning photoelectron microscope according to claim 5,wherein when the ionization and excitation cross section of said gas byelectrons is σ and the molecule density of said gas is N, said electronenergy selecting device is disposed at a location within a value L fromthe sample toward the optical axis of said condensing optical system,where L satisfies the following equation:

    L=1/(N·σ).


7. A scanning photoelectron microscope according to claim 6, furthercomprising a controller for controlling said sample chamber to a gasatmosphere of predetermined pressure.
 8. A scanning photoelectronmicroscope according to claim 7, wherein said controller renders saidsample chamber into a helium atmosphere of predetermined pressure.
 9. Ascanning photoelectron microscope according to claim 7, wherein saidcontroller renders said sample chamber into a nitrogen atmosphere ofpredetermined pressure.
 10. A scanning photoelectron microscopeaccording to claim 6, wherein said light is X-rays, and said scanning isexecuted by movement of said stage.
 11. A scanning photoelectronmicroscope according to claim 5, further including a controller forcontrolling said sample chamber to a gas atmosphere of predeterminedpressure.
 12. A scanning photoelectron microscope according to claim 11,wherein said controller renders said sample chamber into a heliumatmosphere of predetermined pressure.
 13. A scanning photoelectronmicroscope according to claim 11, wherein said controller renders saidsample chamber into a nitrogen atmosphere of predetermined pressure. 14.A scanning photoelectron microscope according to claim 5, wherein saidlight is X-rays, and said scanning is executed by movement of saidstage.
 15. A scanning photoelectron microscope comprising:(a) a samplechamber for containing a sample in a state in which gas is presentaround the sample; (b) an irradiation system which causes photoelectronsto be emitted from the sample; and (c) a detector which detectsphotoelectrons emitted from the sample and multiplied by the gas.
 16. Amicroscope according to claim 15, wherein said irradiation systemincludes a device which causes X-rays to be converged on the sample. 17.A microscope according to claim 15, further comprising:a device whichadjusts a pressure within said chamber.
 18. A scanning photoelectronmicroscope comprising:(a) a system which irradiates a sample so as toproduce photoelectrons from the sample; and (b) a detection system whichdetects photoelectrons produced by the sample through a gas whichmultiplies the photo-electrons.
 19. A microscope according to claim 18,further comprising:an optical device which detects reflected radiationfrom the sample.
 20. A microscope according to claim 19, furthercomprising:a system which displays at least one of a photoelectron imageof the sample obtained by said detection system and an optical image ofthe sample obtained by said optical device.